The cornea is a clear, transparent window of the eye. It contributes two thirds of the eye's focusing power. A healthy cornea should be clear and free of impurities. When cornea tissues are damaged or diseased, the cornea becomes cloudy and vision clarity is reduced. Cornea transplantation is a surgical procedure to replace a damaged or diseased cornea with a healthy one from a donor.
Penetrating Keratoplasty (PKP) or full thickness cornea transplantation is a procedure in which the entire cornea is completely replaced by that of a donor. The original cornea is cut away from the rest of the eye, and the replacement cornea (which has a dome shape) is attached to the remaining portion of the eye by stitches spaced apart around the circular periphery of the replacement cornea. The typical time required for visual improvement is about 6 to 12 months and a custom-made rigid contact lens has to be prescribed for the patient. The risk of rejection is about 8%. Since the cornea transplant wound (i.e. the intersection between the replacement cornea and the remaining portion of the eye) extends throughout the thickness of the replacement cornea, and along its entire circular periphery, the bond between the replacement cornea and the remaining portion of the eye is not strong and is at risk of rupture long after the operation.
An alternative to a PKP is a Descemet's Stripping Automated Endothelial Keratoplasty (DSAEK). This procedure is suitable in cases in which Descemet's membrane (which is a membrane which forms the “lower” surface of the cornea, i.e. the surface facing towards the interior of the eye) is diseased. In this procedure, as illustrated in FIG. 1, a central portion of the diseased Descemet's membrane is removed. This is illustrated in FIG. 1, which shows the eye in cross-section, so that the portion of the Descemet's membrane which is removed in a DSAEK appears as an arc. The portion of the Descemet's membrane is removed through a small incision, and replaced by a healthy one (the “graft”) from a donor. The procedure retains over 90% of the patient's cornea intact and thus has a smaller risk of rejection compared to PKP. Vision improves in roughly 4 to 6 weeks and there is no need for the prescription of a rigid contact lens.
Unfortunately, there is a risk that the graft will become separated from the remaining portions of the original cornea. The graft thickness and whether it is attached properly is of utmost interest for the ophthalmologists. This can be observed using optical coherence tomography (OCT) to examine the graft after a cornea transplant is performed. OCT is a technique using near infrared light to capture 3-dimensional images, effectively behaving like “optical ultrasound”. A 3-dimensional OCT image is composed of a large number of 2-dimensional images, typically scanned in a star-shape formation. FIG. 2 is an OCT image of a patient with a detached graft. The detachment can be observed very clearly. OCT images also show the graft thickness clearly and this allows early detection of possible graft rejection. Thus OCT imagery can assist ophthalmologists to make better decisions.
FIG. 3 shows Visante™ OCT Anterior Segment Imaging System available from Carl Zeiss Meditec Inc. This system provides a means for an ophthalmologist to add virtual calipers manually (i.e. by manipulating a data input device to a computer) to measure the cornea thickness. However, no depth assessment of the transplanted graft is available.